What device is bimetal-actuated and will shut the burner down if it doesn't fire?

The bimetal-actuated device that shuts the burner down if it doesn't fire is the stack switch. This device monitors the temperature of the flue gases in the exhaust stack. If the temperature doesn't rise, indicating that the burner has not ignited, the stack switch will respond to the lack of heat by shutting off the burner to prevent any safety hazards, such as the accumulation of unburned fuel.

The operation of the stack switch is critical for safe burner operation, as it ensures that the system does not continue to attempt to operate in a situation where it cannot ignite, which could lead to potentially dangerous conditions. This safety feature is essential for protecting both the equipment and the space being heated.

In contrast, a flame sensor detects the presence of a flame but does not act based on temperature changes in the stack. A thermostat regulates overall system temperature but does not specifically monitor burner ignition. An exhaust fan facilitates the removal of combustion gases but does not control burner operation in the case of ignition failure.